1. Field of the Invention
The present invention is related to RF and microwave circuits, and particularly circuits using PIN diodes designed by minimizing parasitic capacitance of the PIN diode. The method is not limited to circuit designs that minimize or resonate out diode capacitance. Capacitance could even be deliberately introduced and appropriate inductors employed using the method to control impedance at the diode node to advantage.)
2. Related Art
A P-type, Intrinsic, N-type diode (PIN) is a semiconductor device. PIN diodes are constructed with alternating layers of positively doped, intrinsic, and negatively doped semiconductor material. PIN diodes can be used as RF and microwave signal switches, voltage or current controlled attenuators, and limiters. In these applications, the DC bias condition of the diode controls the effective RE or microwave resistance of the diode.
Increasing voltage amplitude of an RF/microwave signal at higher power levels may cause unwanted changes in the DC condition resulting in undesirable changes in circuit operation. Changes can be caused by forward conduction of a large RF signal that exceeds an applied reverse DC bias. Re-combination of electrical charge carriers in the intrinsic region of a forward biased diode and the associated rectification effect increase for a large RF signal and can also result in changes. Changes can also be caused by reverse voltage breakdown. In addition, high RF voltages can cause significant non-linear changes in parasitic capacitance in the PIN diode under reverse biased conditions. Voltage driven capacitance changes and resistance changes due to recombination can result in unwanted generation of harmonics and inter-modulation distortion, or mixing effects.
Traditional design techniques for RF/microwave PIN diode switches, attenuators and limiters resonate out the parasitic capacitance of the PIN diode device using passive distributed transmission lines, or lumped element techniques. These techniques maintain an impedance match condition at the input and output around a frequency bandwidth.
PIN diodes fabricated with different technologies have different control requirements. For example, PIN diodes fabricated by integrated circuit processes often have lower breakdown voltages than other PIN diodes. PIN diodes fabricated in GaAs or other related materials have higher forward bias voltages than silicon diodes. Furthermore, PIN diodes, like all junction diodes, also have a parasitic capacitance, which is a non-linear function of voltage. This capacitance, which changes with the instantaneous RF or microwave voltage, results in undesirable generation of harmonic frequencies, or sum and difference inter-modulation frequencies. Changes in forward current due to re-combination of charge carriers in the intrinsic region also contribute to harmonic distortion and inter-modulation.
PIN diodes used in switches are generally turned completely OFF in reverse DC bias or completely ON with as much forward DC bias as is practical. In the case of a reverse biased switching diode, the reverse bias DC control voltage must exceed the amplitude of the RF/microwave signal voltage or the diode begins to forward conduct due to the net DC current from the inefficient, yet unavoidable, rectification due to recombination in the intrinsic region. The DC control voltage cannot be increased without limit because reverse breakdown occurs. Reverse biased diodes also exhibit the effects of capacitance modulation by the RF voltage resulting in non-linear effects including generation of harmonics and inter-modulation. Clearly a reduction in RF voltage amplitude for a given power level by a reduction in local impedance can provide significant improvements in operating power range and reduction of harmonic and inter-modulation distortion in the reverse biased diode in a series or shunt PIN diode switch. When the shunt diodes are switched to the ON state with large forward bias, the extra current associated with the lower impedance is not a problem for circuit operation and it can be accommodated by increasing the size of the diode.
PIN diode attenuator circuits employ PIN diodes that are forward biased with specific current levels to achieve a desired effective resistance to the RF/microwave signal. The diodes may be in series, shunt, or both. In general, shunt diodes prefer lower impedances and lower voltages to reduce impedance changes due to rectification and to reduce voltage variable capacitive effects. A partially forward biased series diode should exhibit less rectification and more ideal behavior with the lower forward currents present in a higher impedance environment.
Some limiter circuits consist of diodes with no DC bias applied. As the RF/microwave power in the circuit increases, the forward bias voltage of the diode is exceeded by the RF/microwave signal, and the inefficient rectification due to carrier recombination results a forward current increasing with the RF signal amplitude. The resulting attenuation increases with RF power increase and provides a power-limiting characteristic. The onset of this process occurs at a fixed voltage and RF/microwave power level, related to the forward voltage of the diode. This level may be above or below the desired power level. Control of the RF/microwave voltage, around the limiter diode, could provide some control over the power level where the limiting action occurs.
A number of prior art methods have been created to address the foregoing problems and limitations.
For example, the characteristic impedance might be reduced everywhere in a system, to increase power handling capability. However, reducing the characteristic impedance everywhere does not provide a good method for matching impedances to other components.
An alternative method is to increase voltage and power handling capability by stacking diodes in series. This method could be applied to attenuators, switches, and limiters as well. However, stacking diodes, to increase voltage capability, can be done only in discrete integer steps. The instantaneous voltage splitting across a diode stack cannot be guaranteed without the addition of more components. To create a diode stack a larger area is required and is not as space efficient as a single diode, due to the redundancy of contacts.
A forward or reverse DC bias may be applied to a limiter adjusting its power threshold, either above or below the value based on the diodes constant forward voltage. However, providing for the bias of a limiter requires additional components to isolate the RF signal from the DC connections and requires an additional power supply.
Local control of signal voltage in part of a zero bias limiter circuit may be used. However, the local voltage control or maximization by resonance depends on an earlier set of diodes in the circuit, already being in forward conduction, due to an applied signal. This application is very circuit specific and it is not described or presented as a general technique. Voltage control is only postulated at one node and the reduction of voltage is not explored. The PIN diode may be moved in the signal path or chain to a place where the signal has lower amplitude to control inter-modulation distortion in a PIN diode adjustable attenuator stage. However, moving a PIN diode attenuator in an RF signal path or chain, to reduce inter modulation distortion is often not compatible with other system requirements. Attenuation and re-amplification produces added thermal noise. Linearization techniques and other methods used to correct inter modulation distortion are complicated.
The current state of the art design methodology is unable to provide satisfactory design solution using PIN diodes. Hence, there is a need in the art for an improved design method.